The present invention relates to a control system for four-wheel drive vehicles, and more particularly to a system for controlling the torque capacity of a transfer clutch for four-wheel drive vehicles.
A four-wheel drive vehicle of the type in which a transfer clutch is provided for transmitting the output torque of a transmission to auxiliary drive wheels is known. In such a vehicle, a power transmission system can be converted from a two-wheel drive system to a four-wheel drive system by engaging the transfer clutch. Meanwhile, when the vehicle negotiates corners, braking phenomenon called "tight corner braking" will occur at a sharp corner. This is caused by greater radius of turning of the front wheels than that of rear wheels and therefore tendency to rotate faster than the rear wheels, so that a large torsional torque occurs in the system. Japanese Patent Application Laid Open No. 57-84227 discloses a system which is intended to eliminate the braking phenomenon. In the disclosed system, a four-wheel drive power transmission system is converted to a two-wheel drive power transmission system when a vehicle turns a sharp corner. However, although such a system prevents the tight corner braking at sharp corners by the two-wheel drive vehicle system, characteristics of the four-wheel drive vehicle such as driveabilities on slippery roads, uphill driving and other are not effected.
In order to prevent the tight corner braking while keeping a four-wheel drive power train, it is preferable to employ a friction clutch as a transfer clutch and to control the torque capacity of the transfer clutch so as to slip the clutch in accordance with driving conditions of the vehicle. Further, in order to control the torque capacity, it is advantageous to reduce the torque capacity of the transfer clutch in accordance with the input torque of the clutch. Considering the tight corner braking, at low vehicle speed and large steering angle tight corner braking force becomes large. Generally, in a very low vehicle speed range, an engine clutch for transmitting the output of the engine to a transmission is kept in partial engagement. Accordingly, in partial engagement state of the engine clutch, it is necessary to detect the torque transmitted by the engine clutch and to calculate the torque capacity of the transfer clutch based on the transmitted torque. When the engine clutch is entirely engaged, the torque capacity may be calculated based on engine torque.
On the other hand, there is a transmission having two or more transmission ranges such as a drive range (D-range having an ordinary transmission ratio range) and a large transmission ratio range (D.sub.s -range) in which the transmission is downshifted whereby the vehicle can be actively and powerfully driven. Accordingly, in such a large transmission ratio range, a large torque capacity is preferable for the transfer clutch. Further, when the vehicle is braked, it is desirable to transmit the output of the engine to auxiliary drive wheels irrespective of driving conditions so as to effect engine braking operation on the auxiliary drive wheels, thereby increasing braking characteristics of the vehicle.